This invention relates generally to a portable ionizing, radiation-measuring device of the type used to measure the amount of x-ray and gamma radiation exposure a person receives over a period of time. More particularly, the invention relates to an improved direct reading dosimeter comprised substantially of special plastic material.
The health of personnel handling radioactive materials may be jeopardized if an amount of ionizing radiation of either the electromagnetic or particulate type to which an individual is exposed exceeds that which is permissible. Similar measurements are necessary for measuring the exposure of emergency personnel and the general public to radioactive fallout from the detonation of nuclear weapons, or in the event of a large scale accident at a nuclear power plant or laboratory. The preferred method of measuring these exposures is with a direct reading dosimeter or exposure meter. These devices have an ionization chamber and a fiber electrometer which are externally charged electrically. Incident ionizing radiation passing through the ionization chamber discharges both the ionization chamber and the electrometer. The fiber movement of the electrometer resulting from this discharge is proportional to the amount of ionization produced in the air inside the ionization chamber by radiation passing through it. By using an optical system to measure the amount of movement of the electrometer fiber, an individual wearing the dosimeter on his person is able to accurately determine his own exposure to this radiation. Some of the prior-art dosimeters, because of their relatively heavy weight, have not been entirely successful due to the problem of keeping the fiber position from moving when the instrument is accidentially dropped or subjected to mechanical shock. In many cases when this occured, the fiber position changed, and produced a large increase or decrease in the measurement. Some attempts to correct this problem resulted in a design that could not be charged externally at temperatures below 4.degree. C. This inability to charge the dosimeter can create problems in field applications where charging must be performed outdoors in cold weather.
Another serious problem encountered in prior-art dosimeters is that the response of the electrometer fiber to ionizing radiation is dependent on the direction in which this radiation is impinging on the ionization chamber. Hence, the radiation response is not only dependent on the energy of the electromagnetic radiation but is also dependent on the direction of the radiation.
Many prior-art and dosimeters were of metal and glass. The metals used were usually aluminum alloys. Most of the optical parts were glass. Consequently, metal to glass seals were used, and extensive electro plating or anodizing of various parts to protect their surfaces from oxidation were necessary. Hence, the prior-art processes of dosimeter manufacturing were relatively expensive.